MAMMY
There were three main candidates for this role: Hattie McDaniel who eventually played it, Hattie Noel and Louise Beavers. In the final days of December 1938, in the pre-bbq scene, McDaniel tested with Vivien Leigh and Paulette Goddard, and Hattie McNoel with Joan Bennett and Jean Arthur.

ELLEN O'HARA
Among the very few contenders for this role were Barbara O'Neil, Lillian Gish and Cornelia Otis Skinner. 

CARREEN O'HARA
Selznick wanted to cast Judy Garland as Scarlett's little sister, but Judy was busy working on other classic Wizard of Oz.
Ann Rutherford got the part by coincidence. Once she bumped into David Selznick on a train, and started complaining about how all the stars look ridiculous with plucked eyebrows and advised him not to do such a thing in GWTW. Ann never tested for Carreen and she believes that it was because of this incident. David Selznick in one of his memos mentioned this meeting on the train, but didn't say if it influenced his decision. 

AUNT PITTYPAT
Billie Burke was a strong candidate but eventually was rejected as being too young. 

DOCTOR MEADE
For most of 1937 and 1938 Lionel Barrymore had been a shoo-in before Henry Davenport was cast.

BELLE WATLING
TO: Miss Katherine Brown
December 6, 1938
Would you like to brave the lioness's den and inquire from Miss Tallulah Bankhead whether she would like to play Belle Watling? As a disappointed Scarlett she's likely to bite your head off - and for God's sake don't mention my name in connection with it, simply saying that it is an idea of your own that you haven't yet taken up with me.
My own feeling is that she would do wonders with this bit...however, if she betrays any interest you had better explain that it is an extremely small part, having only about three or four appearances. 
The reason I think she might go for it is simply as a stunt, just as it has been suggested that Mae West (who is out of question of course) might be glad to do it as a stunt.
Do I hear you muttering obscenities? DOS 

BONNIE BLUE BUTLER
This role was originally supposed to be played by Cammie King's older sister, but she had outgrown her character by the time they got around to filming it.